1. Field of the Invention
The present invention relates to a thermally sensitive recording paper that is good at adherent property to a postage stamp or a revenue stamp.
2. Description of the Prior Art
In general, a thermally sensitive recording paper having a thermally sensitive color developing layer mainly composed of colorless or pale colored basic dye and color developer that develops color by reacting with said basic dye when heated, is disclosed in Japanese Patent Publication 45-14039 and is widely used in the commercial scale. A thermal printer in which a thermal head is installed is used for the recording method of said thermally sensitive recording paper. Since this kind of thermal recording method superiors to a conventional recording method from the viewpoints of noiseless at recording process, does not need developing and fixing processes, maintenance free, equipment is relatively cheap and compact and the obtained image is very clear, therefore, this method is widely applied in the field of facsimile or computer, various kinds of measuring instrument and for a labeling machine along with the growth of an information industry.
Recently, the use of a thermally sensitive recording paper for a printing paper of a cash resister is becoming more popular and in many cases a printed-paper by a cash resister is used as a receipt. When a printed-paper by a cash resister is used as a receipt, a revenue stamp is adhered over the surface. Therefore, the requirement to improve the adherent property of a thermally sensitive recording paper to revenue stamp is becoming more severe, namely the adhered stamp must not be removed easily. And the thermally sensitive recording paper having above-mentioned property can be used as a post card, because an adhered postage stamp is not easily removed. However, compared with the normal paper, the adherent property to a postage stamp or a revenue stamp of conventional thermally sensitive recording paper is not so strong, and is easily removed after adhered.
The object of the present invention is to solve the above-mentioned problem that the conventional thermally sensitive recording paper has, and to provide a thermally sensitive recording paper whose adherent property to a revenue stamp or a postage stamp (hereinafter shortened to revenue stamp adaptability) is improved.
The invention disclosed in claim 1 is a thermally sensitive recording paper having a recording surface which when a postage stamp or a revenue stamp is adhered to the recording surface requires at least ten cycles of a removal test procedure to remove at least one third of the adhered surface of the stamp from the recording surface of the thermally sensitive recording paper, wherein the removal test procedure comprises adhering the postage stamp or revenue stamp to the recording surface of the thermally sensitive recording paper, hanging the recording paper over an iron bar of about 8 mm diameter with the postage stamp or revenue stamp on an outer surface of the recording paper, adjusting the recording paper such that equal lengths are on either side of the iron bar, attaching to one end of the thermally sensitive recording paper a weight of about 500 g, holding the other end of the thermally sensitive recording paper by hand and carrying out a pull down ant put back cycle motion of the hand to move the postage stamp or revenue stamp over the iron bar until at least a portion of the postage stamp or revenue stamp is removed from the surface of the recording paper.
In the present invention, the cycles of a removal test procedure to remove at least one third of the adhered surface of the revenue stamp or postage stamp from the recording surface of the thermally sensitive recording paper is an index that indicates adherent degree of adhered revenue stamp or postage stamp. Therefore, if the number of cycles is large, the revenue stamp adaptability is good. And in the present invention, when the number of cycles is over 10 times, the sufficient revenue stamp adaptability can be obtained. Further, when the number of cycles is over 30 times, desirable effect can be obtained and when the number of cycles is over 50 times, more desirable effect can be obtained. On the contrary, when number of cycles is below 10 times, the adhered revenue stamp is easily removed and a thermally sensitive recording paper that has necessary revenue stamp adaptability can not be obtained.
The invention of claim 2 is a thermally sensitive recording paper possessing a middle layer and a thermally sensitive recording layer containing colorless or pale colored basic basic dye and color developer that develops color by reacting with said basic basic dye on a substrate, wherein at least a middle layer between a middle layer and a thermally sensitive recording layer contains polyvinyl alcohol or denatured polyvinyl alcohol whose degree of polymerization is bigger than 450 and degree of saponification is smaller than 90%. When the measuring method disclosed in claim 1 is made on the thermally sensitive recording paper of claim 2, the repeating numbers necessary to remove more than one third of adhered surface area of a revenue stamp becomes over than 10 times.
That is, the inventors of this invention have aimed at a middle layer and have found that the use of polyvinyl alcohol or denatured polyvinyl alcohol whose degree of polymerization is bigger than 450 and degree of saponification is smaller than 90% can improve the revenue stamp adaptability, and have accomplished the invention of claim 2. The inventors of this invention have already found that the revenue stamp adaptability of the thermally sensitive recording paper having a middle layer between a substrate and a thermally sensitive layer is inferior to that of the thermally sensitive recording paper not having a middle layer. The reason why can be considered as follow. That is, usually, a middle layer is prepared for the purpose to improve the color developing sensitivity, therefore the adiabatic property is needed. To satisfy the requirement for high adiabatic property, the porosity of this layer becomes very high. When a revenue stamp is adhered, water stuck to the glue surface of stamp penetrates through the thermally sensitive recording layer and reaches to the middle layer. By the effect of penetrated water, the bonding strength of middle layer is weakened and cracked, then the adhered stamp is removed from the substrate at the crack. Otherwise, it is considered that by the effect of penetrated water, sticking force between middle layer and substrate is weakened, and adhered stamp is removed at the boundary of middle layer and substrate.
A middle layer mainly contains inorganic or organic dye and binder, and as the binder, water-soluble polymer such as polyvinyl alcohol is well known. The type of polyvinyl alcohol used in this invention is the same type that acts usually as a dispersing agent or a bonding agent at the formation of a thermally sensitive recording layer or a protecting layer. Therefore, a lower water-soluble polyvinyl alcohol, that is, full saponificated polyvinyl alcohol whose degree of saponification is high is preferably used. The degree of saponification of polyvinyl alcohol used in general is bigger than 98%. In the meanwhile, since partially saponificated polyvinyl alcohol whose degree of saponification is low has a problem to hinder the color developing ability, it can not be actually used. However, in the present invention, water solubility is improved by containing polyvinyl alcohol whose degree of saponification is lower than 90% or denatured polyvinyl alcohol and can absorb water simultaneously, therefore, adhesive component penetrates into a middle layer with water, and adhere with a revenue stamp strongly after dried up.
Meanwhile, as the conventionally used polyvinyl alcohol, polyvinyl alcohol of lower viscosity, that is, whose degree of polymerization is low is preferably used from the viewpoint of a good dispersing agent or a coating adaptability. On the contrary, it is concerned that the adhering strength of a revenue stamp with adhesive component is improved by containing polyvinyl alcohol or denatured polyvinyl alcohol whose degree of polymerization is bigger than 450. Especially, it is desirable that the degree of polymerization of said polyvinyl alcohol is bigger than 1000. The excellent revenue stamp adaptability can be accomplished by degree of polymerization from 450 to 1000. And, when more strong adhering ability is required, it is effective to use polyvinyl alcohol whose degree of polymerization is bigger than 1000.
It is desirable to contain 1.5-20 weight % of polyvinyl alcohol or denatured polyvinyl alcohol whose degree of polymerization is bigger than 450 and degree of saponification is smaller than 90% to the total solid weight of middle layer. By containing said polyvinyl alcohol by mentioned ratio, excellent revenue stamp adaptability can be obtained. The influence of polyvinyl alcohol to the color developing ability is not so serious because it is contained in a middle layer, but in this invention, the influence of it is more effectively prevented.
In the present invention, polyvinyl alcohol or denatured polyvinyl alcohol whose degree of polymerization is bigger than 450 and degree of saponification is smaller than 90% can be also contained in a thermally sensitive recording layer. Also in this case, the desirable degree of polymerization is bigger than 1000 and the desirable containing ratio is 1.5-10% by weight to the total solid weight of the thermally sensitive recording layer concerning an obstruction to the color developing ability. When the content of polyvinyl alcohol or denatured polyvinyl alcohol whose degree of polymerization is bigger than 450 and degree of saponification is smaller than 90% is too small, the desired revenue stamp adaptability can not be obtained, and when said content is too big, the problem not only obstruction of color developing ability but also deterioration of water proof causes because the solubility to water becomes strong.
The invention of claim 3 is the thermally sensitive recording paper that contains pigment whose oil-absorbing amount is smaller than 100 ml/100 g. When the oil-absorbing ratio of pigment is large, polyvinyl alcohol or denatured polyvinyl alcohol whose degree of polymerization is bigger than 450 and degree of saponification is smaller than 90% is also absorbed and the desired effect to adhering ability with a revenue stamp can not be displayed clearly. In this invention, by containing pigment whose oil-absorbing amount is smaller than 100 ml/100 g, the absorbing amount of these polyvinyl alcohols to pigment can be reduced, and the excellent adhering strength to a revenue stamp can be obtained.